Extrafusal Fiber

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Pacinian Corpuscle as Pressure Transducer

Pacinian Corpuscle as Pressure Transducer

Pacinian Corpuscle Generator Potential

1st node Myelin sheath Lamellated capsule Central core Unmyelinated axon terminal

A. Sharp "on and off" changes in pressure at start and end of pulse applied to lamellated capsule are transmitted to central axon and provoke generator potentials, which in turn may trigger action potentials; there is no response to a slow change in pressure gradient. Pressure at central core and, accordingly, generator potentials are rapidly dissipated by viscoelastic properties of capsule (Action potentials may be blocked by pressure at a node or by drugs)

Capsule Axon

B. In absence of capsule, axon responds to slow as well as to rapid changes in pressure. Generator potential dissipates slowly, and there is no "off" response

Pressure * Na+

Pressure * Na+

Pressure applied to axon terminal directly or via capsule causes increased permeability of membrane to Na+, thus setting up ionic generator current through 1st node

Pressure applied to axon terminal directly or via capsule causes increased permeability of membrane to Na+, thus setting up ionic generator current through 1st node

If resultant depolarization at 1st node is great enough to reach threshold, an action potential appears which is propagated along nerve fiber

If resultant depolarization at 1st node is great enough to reach threshold, an action potential appears which is propagated along nerve fiber

Figure 2.24 Pacinian Corpuscle

Pacinian corpuscles are mechanoreceptors that transduce mechanical forces (displacement, pressure, vibration) into action potentials that are conveyed centrally by afferent nerve fibers. As the viscoelastic lamellae are displaced, the unmyelinated axon terminal membrane's ionic permeability is increased until it is capable of producing a "generator potential." As demonstrated in the figure, pacinian corpuscles respond to the beginning and end of a mechanical force while the concentric lamellae dissipate slow changes in pressure. In the absence of the capsule, the generator potential decays slowly and yields only a single action potential.

Spinal Effector Mechanisms

Spinal Effector Mechanisms

Extrafusal Intrafusal Muscle FibersSpinal Cord Somatotopy

Figure 2.25 Proprioception: Spinal Effector Mechanism

Position sense or proprioception involves input from cutaneous mechanoreceptors, Golgi tendon organs, and muscle spindles (middle figure of upper panel). Both monosynaptic reflex pathways (middle figure of upper panel) and polysynaptic pathways involving several spinal cord segments (top and bottom figures of upper panel) initiate muscle contraction reflexes. The lower panel shows the somatotopic distribution of the motor neuron cell bodies in the ventral horn of the spinal cord that innervate limb muscles (flexor and extensor muscles of upper and lower limbs).

Alpha motor neurons to extrafusal striated muscle end plates

Gamma motor neurons to intrafusal striated muscle end plates

Ia (Aa) fibers from annulospiral endings (proprioception)

II (Ap) fibers from flower spray endings (proprioception); from paciniform corpuscles (pressure) and pacinian corpuscles (pressure)

III (AS) fibers from free nerve endings ^ and from some specialized endings (pain and some pressure)

IV (unmyelinated) fibers from free nerve endings (pain)

Ib (Aa) fibers from Golgi tendon organs (proprioception)-

Alpha motor neurons to extrafusal striated muscle end plates

Gamma motor neurons to intrafusal striated muscle end plates

Ia (Aa) fibers from annulospiral endings (proprioception)

IV (unmyelinated) fibers from free nerve endings (pain)

Ib (Aa) fibers from Golgi tendon organs (proprioception)-

Human Muscle Spindle Golgi

Extrafusal muscle fiber

Intrafusal muscle fibers

Detail of muscle spindle

Efferent fibers Afferent fibers

Figure 2.26 Muscle and Joint Receptors

Extrafusal muscle fiber

Sheath Lymph space Nuclear bag fiber Nuclear chain fiber

Detail of muscle spindle

Intrafusal muscle fibers

Efferent fibers Afferent fibers

Figure 2.26 Muscle and Joint Receptors

Muscle spindles and Golgi tendon organs send afferent signals to the brain to convey the position of limbs and help coordinate muscle movement. Muscle spindles convey information on muscle tension and contraction (dynamic forces) and muscle length (static forces). The nuclear bag fibers respond to both dynamic and static forces, whereas the nuclear chain fibers respond to static forces. Intrafusal fibers maintain appropriate tension on the nuclear bag and nuclear chain fibers. If the muscle tension is too great (e.g., overstretching of muscle or too heavy a load), activation of the Golgi tendon organ causes a reflex relaxation of the muscle.

Ib fibers

Golgi tendon organ

Golgi tendon organ

Intrafusal And Extrafusal

Extrafusal muscle fiber Intrafusal muscle fiber

Alpha motor neurons

Gamma motor neurons

A. Passive stretch. Both intrafusal and extrafusal muscle fibers stretched; spindles activated. Reflex via Ia fibers and alpha motor neurons causes secondary contraction (basis of stretch reflexes, such as knee jerk). Stretch is too weak to activate Golgi tendon organs Ib fibers ++--

Golgi tendon organ

Extrafusal Fiber

Ia fibers

Extrafusal muscle fiber Intrafusal muscle fiber

Inhibitory interneuron

Ia fibers

Extrafusal muscle fiber Intrafusal muscle fiber

Inhibitory interneuron

Alpha activation from brain

Alpha activation from brain

Extrafusal Fiber

Golgi tendon organ

Golgi tendon organ

Golgi Tendon Organ

Gamma motor neurons

B. Active contraction. Central excitation of alpha motor neurons only causes contraction of extrafusal muscle fibers with consequent relaxation of intrafusal fibers; spindles not activated. Tension is low; does not adjust to increased resistance. Tendon organ activated, causing relaxation

Alpha and

Ia fibers + + + + — Extrafusal muscle fiber Intrafusal muscle fiber

Alpha motor neurons

Ia fibers + + + + — Extrafusal muscle fiber Intrafusal muscle fiber

Alpha motor neurons

Alpha Motoneuron

Golgi tendon organ

Gamma motor neurons

C. Active contraction with gamma coactivation. Intrafusal as well as extrafusal fibers contract; spindles activated, reinforcing contraction stimulus via Ia fibers in accord with resistance. Tendon organ activated, causing relaxation if load is too great

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